338 
PROFESSOR W. WILLIAMSON ON THE ORGANIZATION 
exogenous zone, d, in order to reach the bark, as the bundles, m, to, have already done. 
It will be seen that whilst the latter bundles are intersected almost transversely, 
m is almost intersected in the plane of its vessels. Longitudinal sections will explain 
this difference. 
Fig. 34 represents a magnificent section of a stem enlarged nearly three diameters, in 
which the relations of the various parts to each other are well seen. At a we have the 
medullary axis consisting of intermingled cells and vessels ; the former being the 
most abundant in the centre and the latter exclusively forming the peripheral region, 
as seen in fig. 33 c. d is the exogenous layer which forms a cylinder the thickness of 
which is small contrasted with the diameter of the central medullary axis. At g we 
have the delicate parenchyma of the inner bark ; at h the coarser parenchyma of the 
middle bark, and at i is the thin prosenchymatous layer seen in all these Lepidoderi droid 
and Sigillarian stems, and which is generally preserved when all the other tissues have 
disappeared. The persistent bases of the numerous leaves are seen at l. This section 
demonstrates the regularity with which the foliar vascular bundles are given off ; but 
more enlarged figures reveal their relations more clearly. 
Fig. 35 represents a portion of fig. 34 enlarged 20 diameters. At c we perceive the 
large barred vessels of the medullary axis. At d we have the smaller barred vessels 
of the exogenous zone ; at g, g are several portions of the delicate parenchyma 
constituting the inner bark, and which is so frequently more or less destroyed. 
At to we have a bundle of small barred vessels issuing from the exterior of the 
medullary axis, c, and passing obliquely upwards and outwards through the exogenous 
zone, d, and then ascending almost vertically through the inner bark at to', an arrange- 
ment which explains why, in fig. 33, the bundles, to, are intersected transversely, whilst 
to' is cut through more nearly in the plane of its long axis. Both figs. 33 and 35 show 
that the foliar vascular bundle passes outwards accompanied by some cellular paren- 
chyma, either derived from the medullary axis, or more probably from the primitive 
tissue of the bark through which the bundle passed before the exogenous layer began 
to be formed. Fig. 36 represents a tangential section of the thin exogenous vascular 
zone, of which e indicates the vessels, f the numerous small medullary rays, and to, to, 
two lenticular openings through which the foliar vascular bundles have been trans- 
mitted in passing from the outermost medullary vessels, where they originate, to 
the bark and leaves. Fig. 37 represents another portion of fig. 34, but only enlarged 
10 diameters, for the purpose of illustrating the course followed by the foliar 
vascular bundles ; at to one of these bundles is seen passing obliquely upwards and out- 
wards through the exogenous zone as represented in fig. 35. After passing obliquely 
through that zone the bundle bends perpendicularly upwards, running for a little 
distance parallel to, and almost in contact with its external surface, m represents a 
second foliar bundle ascending from the next node below. At its lower extremity, 
like the upper part of to, it ascends parallel with the exogenous cylinder ; but on 
reaching a point a little above that at which the bundle to emerges from the exogenous 
